1. Field of the Invention
The present invention relates to magnetic refrigeration materials and method of manufacturing them, and particularly to magnetic refrigeration materials excellent in magnetic refrigeration characteristics and a method of manufacturing the magnetic refrigeration materials capable of manufacturing the magnetic refrigeration materials with high productivity.
2. Description of the Related Art
In recent years, clean magnetic refrigeration with high energy efficiency ratio is anticipated increasingly as a technology for realizing environment protection type high efficiency refrigeration. Meeting to the requirement as the magnetic refrigeration, magnetic materials exhibiting a large magnetic entropy change at temperature ranges around room temperature have been found out.
Until now, (Hf, Ta)Fe2, (Ti, Sc)Fe2, (Nb, Mo)Fe2, and La(Fe, Si)13 having NaZn13 type crystal structure have been proposed as magnetic materials for the magnetic refrigeration.
Among these magnetic refrigeration materials, materials having NaZn13 type crystal structure and La (Fe, Si)13 type chemical formula have attracted attention. In these materials, Fe mainly occupies the position corresponding to Zn, and La or the like mainly occupies the position corresponding to Na of NaZn13 type crystal structure (after here, these materials are abbreviated as an LaFe13 type magnetic material). These materials having Fe as the main constituent, show promising properties as practical magnetic refrigeration materials providing large magnetic entropy change, and moreover, exhibiting no temperature hysteresis in magnetic phase transition (see, for example, Japanese Patent Laid-open Application No. 2002-356748, Japanese Patent Laid-open Application No. 2003-96547).
There is a report of a method for manufacturing LaFe13 type magnetic material (see X. X. Zhang et al., Appl. Phy. Lett., Vol. 77, No. 19 (2000)). According to the method, LaFe13 type magnetic materials having a phase of NaZn13 type crystal structure (hereinafter, abbreviated as a NaZn13 type crystal structure phase) as the main phase were obtained initially making an unified raw material alloy by melting raw materials using an arc melting method, and then heat-treating the unified alloy at 1000° C. for a long heat-treating time of one month.
At the unifying step of unifying law metals using the arc, high frequency melting method or the like in this process of manufacturing the LaFe13 type magnetic material, the unified alloy contains a large fraction of bcc crystal structure phase comprising Fe as the main constituent (after here, abbreviated as a α-Fe phase), and yield of the NaZn13 type crystal structure phase is hardly seen in the unified alloy. For yielding the LaFe13 type magnetic material from the unified alloy, therefore, the heat treatment for a long time at a high temperature is needed as described above.
Recently, two patent documents, Japanese Patent Laid-open Application No. 2004-100043 and Japanese Patent Laid-open Application No. 2004-99928 concerned with magnetic alloys having the NaZn13 type crystal structure phase containing Fe as the main constituent and their manufacturing methods were published. The first patent document Japanese Patent Laid-open Application No. 2004-100043 discloses a method for producing magnetic alloys controlling formation of stable α-Fe phase and increasing yield of the NaZn13 type crystal structure phase by cooling and solidifying a molten alloy succeeded using a single roll method instead of a conventional self cooling and solidifying method. The intended magnetic alloys were obtained by heat-treating the solidified alloys. The document discloses that this method shortens the time for heat treatment.
In the rapidly cooled alloy obtained using this method, however, the α-Fe phase remains as the main phase. Therefore, heat treatment is indispensable for obtaining an alloy comprising the NaZn13 type crystal structure phase as the main phase. In addition, when the alloy is milled for use as particulate type magnetic refrigeration material, there arises a problem of notable decrease in composition uniformity among the material particles due to existing large amount of α-Fe phase material. Then, there occur particles consisting almost of α-Fe phase, other than the particles composed of NaZn13 type crystal structure phase. Furthermore, with increasing fraction of the α-Fe phase, there happens a problem of increasing difficulty in milling.
It is generally known that cooling speed of a molten metal is about 1×102° C./second in a conventional cooling after melting by a typical method of high-frequency melting or arc melting or the like, and the cooling speed of the molten metal increases up to 1×104° C./second or higher by using a typical rapid liquid cooling method represented by a single roll cooling apparatus. In this specification and claims, cooling at a speed of 1×104° C./second or higher is expressed as forced cooling.
The second patent document Japanese Patent Laid-open Application No. 2004-99928 discloses yield of the NaZn13 type crystal structure phase immediately after casting obtained by comprising 1.8 to 5.4 atomic percent of boron B or the like in the raw material composition. The document further discloses that heat treatment for obtaining the NaZn13 type crystal structure phase is facilitated by comprising B or the like. For the alloys comprising B or the like obtained by casting this method, however, there happens another problem of forming compounds containing B or the like.
Furthermore, A. Yan et al J. Appl. Phys. 97, 036102 (2005) reports structure and magnetic properties of La(Fe, Si)13 prepared by a melt-spinning method. O. Gutfleisch et al J. Appl. Phys. 97, 10M305 (2005) reports a study on large magneto-caloric effect of La (Fe, Si)13 material prepared by a melt-spinning method. Japanese Patent Laid-open Application No. 2005-15911 discloses an invention of material strength enhancement by introducing a phase that structurally reinforces the NaZn13 crystal structure phase of a magnetic refrigeration material. Further, Japanese Patent Application No. 2005-141410 proposes a new production process of a magnetic refrigeration material comprising the NaZn13 type crystal structure phase.